Nonvolatile semiconductor memory devices, which can retain stored data even when power is turned off, include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc.
In the case of EEPROMs, there is a need to manufacture a transistor having a special structure comprised of a floating gate. In the case of FeRAMs and MRAMs, which achieve nonvolatile storage by use of a ferroelectric material and a ferromagnetic material, respectively, there is a need to form and process a film made of these respective materials. The need for such transistor having a special structure and the need for such film made of a special material are one of the factors that result in an increase in the manufacturing costs.
PCT/JP2003/014143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference, discloses a memory cell (i.e., a basic unit of data storage) comprised of a pair of transistors which are configured to experience a hot-carrier effect on purpose for storage of one-bit data. A difference in the transistor characteristics caused by the hot-carrier effect represents one-bit data “0” or “1”.
Specifically, when one of the two transistors is subjected to a hot-carrier effect, a difference in the ON current develops between the two transistors. The difference in the ON current may be detected by a one-bit static memory circuit (latch) coupled to the transistor pair.
The hot-carrier effect is created by high-energy electrons that are generated when an electric current runs through a MOS transistor between the source node and the drain node. It thus follows that the current continuously flows through the MOS transistor at the time of write operation. In order to reduce the time required for write operation effectively, it is preferable to perform write operations with respect to a plurality of bits simultaneously. As long as the hot carrier effect is used as a basis of write operation, however, the number of bits that can be written simultaneously is limited such that a total of the currents flowing through MOS transistors at the time of write operation does not exceed the capacity of power supply.
Accordingly, there is a need for a nonvolatile memory device that can store data based on a change in the transistor characteristics caused by a phenomenon that does not require the flow of an electric current between a source node and a drain node at the time of write operation.